DESCRIPTION: (Applicant's Abstract) Endocytotic membrane retrieval compensates for excess surface membrane following exocytosis but the mechanism of exocytosis-endocytosis coupling is not known. We have shown in sea urchin eggs that membrane retrieval requires calcium influx through agatoxin sensitive channels. Thus, in addition to their role in signaling for exocytosis at synapses, P-type calcium channels are required for endocytotic membrane retrieval in eggs. We hypothesize that exocytosis regulates P-type calcium channel gating to coordinate exocytosis and endocytotic membrane retrieval. We will use microscopy, electrophysiology, as well as cell and molecular biological techniques to determine how exocytotic activity regulates calcium influx through P-type channels in sea urchin eggs, a model system for understanding calcium-triggered exocytosis and endocytosis. Specifically we will determine how exocytotic activity influences membrane depolarization and the cellular distribution of P-type calcium channels.